IWYU: Suppress warnings from the internal Clang

Do not obscure IWYU's report with warnings from its internal Clang.
We have other testing for such warnings.  Also, when compiling with
a non-Clang compiler we might use warning options that IWYU's Clang
does not understand, and we don't want to see warnings about that.

CMakeLists.txt

@@ -267,7 +267,7 @@ if(CMake_RUN_IWYU)
     message(FATAL_ERROR "CMake_RUN_IWYU is ON but include-what-you-use is not found!")
   endif()
   set(CMAKE_CXX_INCLUDE_WHAT_YOU_USE
-    "${IWYU_COMMAND};-Xiwyu;--mapping_file=${CMake_SOURCE_DIR}/Utilities/IWYU/mapping.imp")
+    "${IWYU_COMMAND};-Xiwyu;--mapping_file=${CMake_SOURCE_DIR}/Utilities/IWYU/mapping.imp;-w")
 endif()